With an increase in communication traffic in recent years, the demand for optical communication systems has increased. This type of an optical communication system includes a wavelength multiplexing optical amplifier (e.g., erbium doped fiber amplifier (EDFA)) for each of the transmission paths. The wavelength multiplexing optical amplifier amplifies signal light that propagates in the transmission path. A high-capacity optical amplification relay system that achieves long-distance transmission is a mainstream system.
In the optical amplification relay system, when transmission loss is large due to a long transmission path or the like, the power of signal light to be input to the EDFA is reduced. Thus, a transmission characteristic may be degraded due to a reduction in an S/N ratio that is the ratio of the power of the signal light to the power of noise light. To avoid the degradation of the transmission characteristic, Japanese Unexamined Patent Application Publications Nos. 2009-239813 and 2005-277044 each describe a distributed Raman amplification technique in which pump light is injected into the transmission path and an amplification effect is used with an effect of induced Raman scattering. The S/N ratio is increased by increasing the level of the signal light to be input to the optical amplifier. Thus, the transmission characteristic may be improved, and the number of spans available for transmission and a transmission distance may be increased.
In order to perform the Raman amplification and compensate for transmission loss in the transmission path, it is beneficial and/or necessary to accurately measure the transmission loss in the transmission path of the signal light. However, when the transmission path is long and causes the degradation of the S/N ratio or the like, the power of the signal light output from the transmission path may be difficult and/or impossible to appropriately measure. Therefore, there is a technical problem that the transmission loss may not be accurately measured. In this case, an amplification effect that uses Raman amplification and is necessary to enable the signal light to be reliably transmitted may not be determined, and the amplification may not be appropriately performed.
Japanese Unexamined Patent Application Publications Nos. 2009-239813 and 2005-277044 each describe a configuration in which monitoring light that is different from the signal light is supplied into the transmission path and the transmission loss is measured using the monitoring light. However, in order to achieve the configuration, it is necessary to prepare a high-power monitoring light source that supports the transmission loss caused by the long transmission path. Thus, a load is applied to a system that has the configuration. In addition, injection of the monitoring light into a power monitor located on the downstream side of the transmission path may cause a technical problem, or may reduce the accuracy of detecting the power of the signal light.